Florence's octagonal baptistery is one of the most important creations of the so-called Tuscan proto-Renaissance, which is typified by marble exterior sheathing, a rich wall arrangement, and sculptural architectural decor based on structures from antiquity. The decoration of the baptistery's interior is no less ambitious, it includes an extensive mosaic decor, undertaken in around 1240-50.

The octagonal space is roofed by an eight-sided cloister vault, on whose faces the mosaics are arranged in six horizontal registers, the top two filled with ornamental motifs and single figures, the lower ones featuring complete scenes. This scheme is abandoned only in the three vaulting segments of the west side in which the number of registers is reduced from six to five and the division into registers is interrupted for nearly the entire height by a large figure of Christ as World Judge in a circular aureole that dominate the impression of the whole. The Last Judgment is pictured next to Christ.

Aside from the Last Judgment, the pictorial program consists mainly of the biblical stories long traditional in Rome. The inclusion of a cycle on the life of the church's patron - here St John the Baptist - had Roman precedents as well, as do the various decorative motifs stretching across the top of the vault.

Roughly sixty to seventy years were required to complete the vaulting mosaics. This long period explains the stylistic differences that can easily be seen in the mosaics, some of them significant. The representations offer insight into the popular style then current in Florence, a mixture of the local late Romanesque and an imported Byzantine revival.

We do not know precisely when the planning and execution of the mosaics were begun and when the work was completed. As for the artists working on the project, almost all the painters who were active in Florence in the period in question have been somewhat associated with the baptistery mosaics. The oldest mosaics are usually attributed to such artists as Coppo di Marcovaldo, his son Salerno, and a certain Meliore. Moreover, a Luccan painter from the circle around Bonaventura Berlighieri and the Master of San Francesco Bardi are also mentioned.